Our long-term objective is to achieve an understanding of the molecular properties of the estrogen receptor and its role in regulating normal and abnormal uterine growth and reproductive function. We have described the mechanism by which estradiol-binding modulates the equilibrium between the nonactive and the active states of the estrogen receptor. The estrogen receptor exists in two forms or states: the nonactive, 4S monomer, which has a lower affinity for estradiol and rapid estradiol dissociation kinetics; and the activated, 5S receptor dimer, which has a higher affinity for estradiol and slower estradiol dissociation kinetics. Equilibrium between these two forms of the estrogen receptor induces a posibively cooperative equilibrium binding of estradiol. Since there is only limited information on several crucial aspects of the biochemistry of the estrogen receptor, we propose: 1) A careful kinetic and thermodynamic analysis of the interactions between the receptor and tritiated and nontriatiated potent agonists (estradiol, diethylstilbestrol), weak estrogens (estrone, estroil) and estrogenic antagonists (clomiphene, tamoxifen, hydroxytamoxifen, and LY117018). The relationship between the receptor's binding kinetics with each of these compounds and their biological activity will be compared. 2) Characterization and kinetics of the DNA binding properties of activated as compared with nonactivated forms of the estrogen receptor. The role of estrogen-binding by the receptor on the receptor's DNA binding interactions will be investigated. 3) Estrogen affinity chromatography will be used to purify the estrogen receptor from calf uteri, and its molecular properties will then be described. 4) Monoclonal antibodies will be prepared as a probe for the receptor's biochemistry.